Divalent nickel carboxylates derived from short-chain acids, i.e. having less than 5 carbon atoms, such as acetate, propionate, butyrate and their halogenated derivatives such as chloroacetate, dichloroacetate, trichloroacetate, trifluoroacetate, are known as being practically insoluble in hydrocarbons under normal temperature conditions. On the contrary, the divalent nickel carboxylates derived from long-chain acids, i.e. comprising more than 5 carbon atoms and preferably having a branched chain, such for example as nickel 2-ethyl hexanoate, are soluble in hydrocarbons. The U.S. Pat. No. 4,316,851 discloses means for solubilizing a halogenoacetate anion linked to nickel, by use of a mixed salt wherein nickel is linked by one of its valencies to the halogenoacetate anion and by the other to the anion of a long-chain carboxylic acid, i.e. comprising at least 5 carbon atoms.
However the presence in a nickel halogenoacetic derivative of an anion derived from a long-chain carboxylic acid results in an additional manufacturing cost and may result in disadvantages when in use. For example, the nickel salts containing the halogenoacetate anion are reacted with alkylaluminum halides in hydrocarbon solution to catalyze the dimerization of olefins in liquid phase; one of the means to remove the inorganic compounds present in the medium at the end of the dimerization step consists of extracting them with a sodium hydroxide aqueous solution; during this step, the presence of the long-chain carboxylate anion, as a result of the surfactant properties of its sodium derivative, leads to the formation of foams, which are detrimental to the phase separation, and also to poor dispersion of the nickel oxide produced by reaction of the catalyst with sodium hydroxide.